This application is directed to an improved vibration absorber for individual suspended cables including but not limited to electrical transmission lines; and more specifically to a device for absorbing energy to suppress aeolian vibration of such cables.
The vibration absorber herein described operates on the principles disclosed in co-pending U.S. Patent Application Ser. No. 147,096, filed May 7th, 1980 (now issued as U.S. Pat. No. 4,346,255), assigned to the assignee of the present application and entitled "overhead electrical conductor system including SUBSPAN OSCILLATION AND AEOLIAN VIBRATION ABSORBER FOR SINGLE AND BUNDLE CONDUCTORS". This prior application discloses and claims an essentially dissipative (as opposed to spring-type) vibration absorber having a damping mechanical impedance which essentially matches the mechanical impedance of the transmission line to which the damper is attached. The acceptable range of damper impedance of the absorber is indicated as being anywhere between half and three times the transmission line mechanical impedance.
By essentially matching the transmission line mechanical impedance, and providing a dissipative (i.e., frequency independent) damping effect, the vibration absorber of the aforesaid U.S. Patent Application Ser. No. 147,096 provides optimum energy coupling between the transmission line and absorber, thus effectively absorbing travelling waves on the line before they build up to large amplitude standing waves which can cause damage to the line and associated supporting elements.
Until the aforesaid invention was made, prior art spring-type dampers had to be designed so that they operated effectively over the resonant frequency range of the transmission line to be damped. These dampers also had to be situated at points on the transmission line where standing waves would be of relatively large amplitude, i.e., at distances of a quarter wavelength from adjacent nodes.
The invention of U.S. Patent Appln. Ser. No. 147,096 relates to the use of vibration absorbers which can be connected to transmission lines to provide essentially dissipative damping. That is, these dampers utilize viscous-type effects, so that damping is essentially frequency independent. In contradistinction, those prior art dampers which utilized springs or other resilient elements had undesirable resonance characteristics. A typical prior art vibration damper of this type is shown in U.S. Pat. No. 3,885,086. The vibration damper shown in this patent, however, is unsuitable for use in the arrangement contemplated by the aforementioned U.S. Patent Appln. Ser. No. 147,096, because it is incapable of providing the critical dissipative damping required. In U.S. Pat. No. 3,885,086, the annular washers 12 are situated between clamp arms 16 and adjacent frame portions 10, and secured thereto so that said washers do not rotate. The washers are of a resilient material, so that rotation of the clamp arm 16 results in deformation of the washers, the resilient characteristics of which then return the clamp arms to their initial orientations. In this arrangement, the only dissipative damping effects are provided by hysteresis losses within the resilient washers.
Any attempt to increase the hysteresis losses by increasing the size of the washers results in the spring force of the washers rising substantially faster than their hysteresis losses, making such a design impractical. Further, limitations of the resilient material itself make it impracticable to obtain sufficiently great hysteresis losses to provide critical dissipative damping. In addition, the hysteresis losses in the washers 12 are dependent upon both frequency and amplitude of vibration.
Vibration absorbers intended for use with single cables or conductors utilize a mass which acts as a platform to induce oscillation within the vibration absorber. Where the cable or conductor is stranded and therefore exhibits a high level of torsional damping (due to interstrand friction), torsional vibration absorbers may be employed to reflect the vertical aeolian vibrations into torsional conductor oscillations. Such torsional vibration absorbers are described in Canadian Pat. Nos. 377,602; 546,134; 559,081; 567,131; and 570,780; and in the following articles:
1. Measurement and Control of Conductor Vibration by Gordon B. Tebo AIEE Transactions Volume 60-1941 pp 1188-1193 PA1 2. Conductor Vibration--Theory of Torsional Dampers by James W. Speight AIEE Transactions Volume 60-1941 pp 907-911
Canadian Pat. No. 570,780 utilizes rubber washers to internally damp the vibration absorber. In common with the other prior art single cable vibration absorbers, however, this structure provides poor damping action since it has a high spring constant and has no means for positive damping, relying solely on hysteresis losses in the rubber.
The aforementioned prior art vibration absorbers have a limited ability to dissipate energy, pronounced resonances, and a limited effective frequency range. They also suffer from incompatibility between the spring constant required to restore the vibration absorber to its neutral position and the rubber (or other resilient material) characteristics required to provide the optimum mechanical impedance. That is, a compliance low enough to provide proper restoration to the neutral position results in an undesirably high mechanical impedance which reflects (rather than absorbing) a considerable amount of aeolian vibration energy back to the cable supports at the suspension points thereof.
In contradistinction, dampers of U.S. Patent Appln. Ser. No. 147,096, being dissipative and therefore frequency-independent, need not be concerned with the resonant frequencies of the transmission line to which they are to be attached. Further, such dampers, being essentially impedance matched (i.e., within a range of one-half to three times the characteristic impedance of the transmission line to which they are to be attached), absorb travelling waves, so that they can be placed at any desired place on the transmission line to be damped.
Thus, there remains a need for an improved vibration absorber capable of being utilized with individual suspended cables or conductors according to the aforementioned principles.
Accordingly, an object of the present invention is to provide an improved vibration absorber for individual suspended cables or conductors in which the damping effect produced is essentially dissipative and therefore frequency-independent, and wherein the damping impedance is adjustable to a value capable of essentially matching the mechanical characteristic impedance of the cable to which the vibration absorber is connected.